EP0010926B1

EP0010926B1 - Input scanner using a two dimensional detector array

Info

Publication number: EP0010926B1
Application number: EP79302327A
Authority: EP
Inventors: Robert Arthur Sprague
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1978-10-25
Filing date: 1979-10-25
Publication date: 1982-12-29
Also published as: US4204230A; JPS6364114B2; EP0010926A1; CA1136275A; DE2964434D1; JPS5560383A

Description

This invention relates to an input scanner comprising a two-dimensional array of photosensitive detectors, imaging means for imaging scan line length segments of a subject onto the array, and readout means coupled to the array for reading data samples out of the detectors, wherein the detectors in the array have respective photosensitive zones which are laterally offset from one another in the line scanning direction.
Others have already recognized that charge coupled devices (CCD's) may be advantageously utilized as photosensitive detector elements for raster input scanners. It has been shown that many mutually independent CCD's can be formed on a single chip of semiconductive material, such as silicon. Nevertheless, the chip lengths and the CCD detector densities which can be obtained using state of the art semiconductor fabrication techniques are still not sufficient to permit the manufacture of an integrated linear CCD detector array having the high line scanning resolution capability demanded of some raster input scanners.
In view of that limitation, some thought has been given to the seemingly simple expedient of stringing a plurality of linear integrated CCD arrays together to form a longer, composite linear detector array. However, it has been found that it is difficult to achieve and maintain the alignment of the individual integrated arrays that is essential to the linearity of the composite array. Another suggestion which has been made for applying integrated CCD detector arrays to high resolution raster input scanning involves optically interlacing or stitching the detector elements within several rows of a two dimensional integrated array to perform the scanning. See, for example, U.S. Patent No. 4,080,633 of Gary K. Starkweather, which issued March 21 1978 to the assignee of this application. That is an effective approach, but it suffers from the disadvantage of requiring relatively complex and difficult to align optics for imaging the subject which is to be scanned onto the detector array. Another example of the use of a two-dimensional CCD array is found in U.K. Patent Specification No. 1 501 017, which describes a television camera incorporating such an array.
The input scanner of the present invention is intended to overcome the disadvantages of known scanners, and to provide a high-resolution scanner using a low resolution, two dimensional image sensor array such as a CCD area array. It is characterised in that the imaging means is arranged for sequentially imaging successive scan line length segments of the subject onto the array at a predetermined framing rate as the subject moves in a cross-scan direction relative to the array, and in that all the detectors in the array are accessed to provide signal components collectively constituting an analysis scan line signal for each scan line.
In an input scanner embodying this invention, the subject to be scanned is translated in a cross scan direction rotative to a two dimensional, integrated CCD detector array or the like so that successive full scan line length segments of the subject are sequentially imaged onto the array. The photosensitive zones of the detectors are laterally staggered within the different rows of the array so that each of the detectors responds to a specific spatially predetermined resolution element or pixel of each scan line. The detectors generate data samples in response to those pixels, but the data samples representing adjacent pixels of any given scan line are distributed over multiple rows of the array in accordance with a two dimensional distribution function which depends on the staggering of the photosensitive zones of the detectors. If multiple scan lines are simultaneously imaged onto the array, buffered de-staggering electronics, operating in accordance with the inverse of that distribution function, may be utilized to arrange the data samples representing adjacent pixels in a sequential manner, thereby providing the scan line-by-line serial video data stream output format of a conventional raster input scanner.
The basic advantage of this invention is that relatively high resolution input scanning can be achieved through the use of relatively low resolution, two dimensional photosensitive detector arrays. This is particularly important if it is desired to use a two dimensional, integrated CCD array because the detector density which may be achieved in such an array is limited by the state of the art fabrication processes. Nevertheless, certain of the basic features of this invention are applicable to input scanners which utilize other types of two dimensional image sensors, such as vidicons.
Embodiments of an input scanner in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic illustration of one embodiment of the present invention;
Figure 2 is a schematic illustration of another embodiment of this invention;
Figures 3-7 are layout diagrams of staggered aperture masks which are suitable for use in the embodiments shown in Figures 1 and 2;
Figure 8 is a schematic diagram of still another embodiment of the present invention;
Figure 9 is a layout diagram of a staggered aperture mask which is suitable for use in the embodiment shown in Figure 7;
Figure 10 is a functional block diagram of buffered destaggering electronics for carrying out raster input scanning when the mask of Figure 9 is used in the embodiment of Figure 8; and
Figure 11 is a functional diagram of a fully integrated equivalent to the buffered destaggering electronics illustrated in Figure 10. Turning now the drawings, and at this point especially to Figure 1, there is a lens 21 for sequentially imaging successive full scan line length segments of a suitably illuminated subject 22 onto a two dimensional integrated CCD detector array 23 via a staggered aperture mask 24. To position the successive segments of the subject 22 for imaging onto the array 23 at a predetermined framing rate, the subject is moved at a predetermined rate (by means not shown) in a cross scan direction relative to the array 23, as indicated by the arrow. As a general rule, the width of the subject 22, as measured in the line scanning direction, is substantially greater than the length of the array 23, as measured in that same direction. Thus, the magnification of the lens 21 is selected so that full width or scan line length segments of the subject 22 are imaged onto the array 23. The array 23 is at least an nxN array of CCD's. In other words, it comprises N more or less equidistantly spaced and mutually independent CCD detectors 25 in each of n successive rows. In the illustrated embodiments, the detectors 25 are integrated on a single chip of semiconductive material, such as silicon. Accordingly, the number N of mutually independent CCD detectors 25 which may be provided in each row of the array 23 is limited by the maximum chip length and the maximum detector density which can be achieved through the use of state of the art semiconductor fabrication techniques.

More than enough resolution elements or pixels to provide a high resolution definition of a scan line are imaged onto a single row of detectors 25. However, the limited number N of CCD detectors 25 which may be formed in any one row of the array 23 is insufficient to obtain high resolution input scanning through the use of just one row of detectors 25.
In accordance with the present invention, to perform high resolution input scanning despite the characteristically low line resolution capabilities of the array 23, the photosensitive zones of the detectors 25 are laterally staggered or offset from one another in the line scanning direction so that individual pixels from each scan line are separately imaged onto respective ones of the detectors 25 in one or another of the rows of the array 23. That, of course, greatly increases the number of pixels which are detected and converted into corresponding data samples per scan line, thereby providing increased line scanning resolution. The basic concept is unaffected by whether the individual pixels which are imaged onto the detectors 25 are from abutting segments of the scan line as in a fully sampled case, overlapping segments of the scan line as in an oversampled case, or spaced apart segments of the scan line in an undersample case.
The individual pixels of each scan line are distributed while being imaged onto the detectors 25 of the array in accordance with a two dimensional distribution function which is dependent on the staggering of the photosensitive zones of the detectors 25..Pixels of multiple scan lines may be simultaneously imaged onto different ones of the detectors 25, but all of the pixels of each scan line are detected within a predetermined number of frames. Each of the detectors 25 responds to a spatially predetermined one of the pixels of each scan line. Consequently, clock pulses (supplied by means not shown) are applied to the array 23 to shift or read out the data samples generated by the detectors 25 at the line scanning or framing rate. Multiple clock pulses may be supplied per frame to serially read out the data samples from the detectors 25 in successive columns or rows of the array 23. Alternatively, there may be one clock pulse per frame to simultaneously read out the data samples from all the detectors 25 in parallel. In either case, the number of frames required to accumulate a complete set of data samples for a scan line is dependent on the number of scan lines which are imaged onto the array 23 per frame.
In keeping with this invention, to laterally stagger the photosensitive zones of the detectors 25 within the successive rows of the array 23, the mask 24 is an optically opaque screen which is formed to provide laterally staggered, optically transparent apertures 31 (Figs. 3-6) in alignment with respective ones of the detectors 25 in successive rows of the array 23. As shown in Figure 1, the mask 24 may be a metallized layer deposited directly on the array 23. Alternatively, as shown in Figure 2, the mask 24' may be a free standing component which is relay imaged onto the array 23 by the imaging lens 31 and suitable relay optics 32. Indeed, there are several other approaches which can be used for laterally staggering the photosensitive zones of the detectors 25 within the successive rows of the array 23. For example, the boundary zones between the detectors 25 in successive -rows of the array 23 could be staggered and the width of the boundary zones could be controlled by applying a suitable biasing voltage to control the width of the photosensitive zones of the detectors 25. In other words, the masks 24 and 24' simply represent one technique for laterally staggering the photosensitive zones of the detectors 25 in the successive rows of the array 23. Nevertheless, it should be noted that the advantage of relay imaging the mask 24' onto the array 23 is that the mask 24' may be substantially larger than the ₆gray 23, thereby simplifying the mask fabrication process, par- titularly for higher resolution applications.
Figures 3_-5 illustrate some of the staggered aperture patterns which may be used with the mask 24 of Figure 1. Of course, the same patterns are applicable to the mask 24' of Figure 2 and to any of the other techniques which can be employed to laterally stagger the photosensitive zones of the detectors 25.
The progressive staggering pattern shown in Figure 3 causes the adjacent pixels of a scan line to be imaged onto detectors 25 in adjacent rows of the array 21. It simplifies the destaggering electronics which are required to achieve a raster scanning format, but it is not optimal from the standpoint of its sensitivity to alignment errors or scanning errors. If alignment errors, such as an angular error in the motion of the subject 22 relative to the array 23, are of primary concern, the pattern illustrated in Figure 4 is especially attractive because it minimizes the average cross scan diplacement of apertures 31 which are adjacent in the line scanning direction. On the other hand, if visual banding in the output image (not shown) is particularly objectionable, a pseudo-random staggering pattern, such as is shown in Figure 5, may be desirable because it causes any line scanning errors to be more or less randomly distributed over several rows in the data. In short, the lateral staggering of pattern for the photosensitive zones of the detectors 25 in the successive rows of the array 23 may be selected to accomodate any one of several different requirements.
Figures 3-5 illustrate staggering patterns in which the edges of respective pairs of apertures 31 for detectors 25 in successive rows of the array 23 are aligned in the cross scan direction for a so-called fully sampled application. It should, however, be understood that the apertures 31 may be paired in overlapping edge alignment for over sampling or in spaced apart edge alignment for under sampling.
A common disadvantage of using a simple rectangular array 23 of CCD detectors 25 such as shown in Figures 1 and 2, is that some of the pixels of each scan line are likely to be imaged onto certain of the detectors 25 in regions or zones which are too close to the detector boundaries. However, that problem may be avoided, as shown in Figure 6, by laterally staggering the detectors 25' in the successive rows of the array 23' so that the photosensitive zones are generally centered on the detectors 25' despite the lateral staggering of the apertures 31. Altematively, as shown in Figure 7, a rectangular CCD detector array 23 may be canted or tilted relative to the cross scan direction to more or less center the staggered apertures 31 on the individual detectors 25. In that event, pixels from multiple scan lines of the subject may be simultaneously imaged onto each row of detectors 25, but that does not alter the basic concept.
Referring to Figure 8, to reduce the number of frames which are required to accumulate a complete set of data samples for a scan line, anamorphic optics 36 having greater magnification in the cross scan direction than in the line scanning direction may be used for imaging the subject 22 onto the array 23. The complete optical system, including the lens 31 and the optics 36, is suitably selected so that cross scan height of each scan line imaged onto the array 23 is in a range anywhere between a cross scan height equal to -the line scan pixel spacing (the nonanamorphic case) and a cross scan height equal to the height of the entire array 23. In the example shown in Figure 8, the cross scan or vertical height of each scan line is equal to the cross scan height of the array 23 per row of detectors 25. It will, of course, be understood that it becomes increasingly difficult to achieve the f/numbers needed for high speed scanning as the anamorphic ratio of the optical system is increased.
As will be appreciated, the line scanning resolution cannot be increasing indefinitely simply by adding additional rows of detectors 25 to the array 23. In an integrated array 23, the number of rows n of detectors 25 is subject to the aforementioned limitations on state of the art semiconductor fabrication techniques. However, the line scanning resolution which can be achieved is generally even more restrictively limited since each of the detectors 25 requires a photosensitive zone of at least a certain minimum area to provide sufficient photosensitive response at typical illumination levels.
Turning to Figure 9, one of the additional advantages of anamorphically imaging the successive segments of the subject 22 is that the areas of the photosensitive zones of the detectors 25 may be anamorphically enlarged in the cross scan direction, such as by providing the mask 24 with anamorphic apertures 31'. Preferably, the anamorphic ratio of the photosensitive zones of the detectors 25 is related to the anamorphic ratio of the optics 31, 36, up to a limit determined by the cross scan height of the individual detectors 25. That maximizes the area per unit width of the photosensitive zones of the detectors 25, while ensuring that each of the detectors 25 responds to only one pixel per scan line.
The present invention may be applied to raster input scanning if suitable provision is made for assembling the data samples generated by the detectors 25 in a serial video data stream having a scan line-by-scan line format. As previously mentioned, the pixels of each scan line are distributed to the individual detectors 25 of the array 23 in accordance with a two dimensional distribution function. If only one scan line is imaged onto the array 23 per frame, the detectors 25 simultaneously generates data samples for the same scan line. In that case, it is merely necessary to provide suitable destaggering means for assembling the data samples for each scan line in accordance with the inverse of the aforementioned distribution function. If, however, multiple scan lines are simultaneously imaged onto the array, the destaggering means must be buffered. The minimum data sample storage capacity, M, required for that buffering is given by:
Where:

N=the number of detectors 25 per row in the array 23;
x=the total number of undetected scan lines imaged onto the array 23 between the uppermost and lowermost rows of detectors 25; and
n=the number of rows in the array 23.

Referring to Figure 10, there is a functional block diagram of a buffered destaggering circuit which may be used to provide a video data stream having a raster scanning format if (1) the photosensitive zones of the detectors 25 are progressively staggered and (2) n scan lines are imaged onto the array per frame (i.e., x=0). In this instance, it is assumed that the data samples generated by the detectors 25 in the successive rows of the array 23 are serially shifted along the respective rows and out of the array in parallel, with each row being read out at the line scanning or framing rate. Data samples from the last or uppermost row of detectors 25 are serially shifted into the last stage of a n stage parallel input/serial output shift register 41, but the data samples from the other rows of detectors 25 are shifted into other stages of the register 41 via buffer registers 42-44, respectively, of progressively increasing length. Specifically, N additional stages are added to the registers 42-44 for each succeeding row in the array 23. Thus, the buffer register 42 for the next to last row of detectors 25 has N stages, while the buffer register 44 for the first or bottommost row of detectors 25 has N (n-1) stages.
Turning to Figure 11, an extended CCD array 51 may be used to carry out the image detection function and the buffered destaggering function. Moreover, the extended array 51 may be integrated on a single. semiconductor chip, provided that the required CCD element count does not exceed the capabilities of state of the art fabrication techniques. For example, as shown, if n scan lines are imaged onto a nxN image detection segment 52 of the array 51 per frame via a mask 24 having progressively staggered apertures 31, a suitable shift sequence for assembling the data samples for a given scan line in adjacent sample serial order is:

1. Shift all data samples down n rows per frame;
2. Shift the n+1, 2n+2,... nn+n rows one stage to the left (it should be noted that each of those rows contain N+ 1 CCD's as opposed to the others which contain only N CCD's);
3. Shift the contents of the leftmost stages of the n+1, 2n+2, ... nn+n rows down N elements;
4. Repeat steps 2 and 3, n-1 additional times/frame to empty the n+1, 2n+2 and nn+n rows; and
5. Go back to step 1 to start assembling the data samples for the next scan line.

In view of the foregoing, it_;will be seen that the present invention provides methods and means for achieving high resolution input scanning, including raster input scanning, through the use of a low resolution, two dimensional, photosensitive detector array, such as CCD area array. Furthermore, it will be understood that this invention may be used for scanning a variety of subjects, such as printed or handwritten documents, graphics, photographs, or even real scenes.

Claims

1. An input scanner comprising a two-dimensional array (23) of photosensitive detectors (25), imaging means (21 or 31, 32) for imaging scan line length segments of a subject (22) onto the array (23), and read out means coupled to the array (23) for reading data samples out of the detectors (25) wherein the detectors (25) in the array (23) have respective photosensitive zones which are laterally offset from one another in the line scanning direction, characterised in that the imaging means (21, or 31, 32) is arranged for sequentially imaging successive scan line length segments of the subject (22) onto the array at a predetermined framing rate as the subject moves in a cross-scan direction relative to the array, and in that all the detectors in the array are accessed to provide signal components collectively constituting an analysis scan line signal for each scan line.

2. The input scanner of claim 1 wherein each of said segments of said subject (22) comprises a predetermined number of scan lines, and said subject (22) advances in the cross scan direction relative to said array (23) at a rate of one scan line per frame.

3. The input scanner of claim 2, wherein said imaging means (21 or 31, 32) includes anamorphic optics having greater power in said cross scan direction than in said line scanning direction, said optics having an anamorphic ratio selected to cause each of said scan lines to have a cross scan height greater than the width of any of said pixels in the line scanning direction, and the photosensitive zones of said detectors (25) being elongated in the cross scan direction to provide said detectors (25) with increased sensitivity per unit width of said photosensitive zones as measured in said scanning direction.

4. The input scanner of aný one of claims 1 to 3, wherein a staggered aperture mask (24 or 24') is interposed between said subject (22) and said array (23) for laterally offsetting the photosensitive zones of said detectors (25).

5. The input scanner of Claim 4 wherein the photosensitive zone of each detector (25) is generally centered on the detector (25).

6. The input scanner of Claim 4 or Claim 5 wherein said staggered aperture mask (24) is deposited on and supported by said array (23).

7. The input scanner of Claim 4 or Claim 5 wherein said staggered aperture mask (24') is a free standing component, and said imaging means further including relay optics (32) for Imaging said mask (24') onto said array.

8. The input scanner of Claim 6 or Claim 7 including buffered destaggering means (41-44 or 51) coupled to said detectors (25) for assembling the data samples read out of said detectors in a serial video data stream having a raster scan format.

9. The input scanner of Claim 8 wherein said detectors (25) and said buffered destaggering means (51) are respective pluralities of charge coupled devices integrated on a single chip of semi-conductor material.

10. The input scanner of any one of Claims 1 to 9 wherein said detectors (25) are photosensitive devices integrated on a single chip (23) of semi-conductor material.